The present invention relates generally to controlling the level of liquid in a condenser of a chiller system. More specifically, the present invention relates to using fuzzy logic controls to properly position (open/close) a valve between the condenser and the evaporator to regulate the level of liquid in the condenser and to restrict the flow of refrigerant vapor to the evaporator.
In a chiller system, refrigerant gas is compressed in a compressor and delivered to a condenser where it is condensed to a liquid refrigerant. The liquid refrigerant from the condenser passes through an expansion device to lower the pressure of the refrigerant liquid before reaching an evaporator. In the evaporator, the liquid refrigerant is vaporized and returned to a suction inlet of the compressor to repeat the process.
The expansion device can incorporate a valve to regulate the flow of refrigerant between the condenser and the evaporator. The flow of refrigerant (liquid or gas) through the valve is dependent upon the pressures in the condenser and the evaporator and on the geometry and positioning of the valve. Ideally, the valve is positioned such that the resistance to fluid flow in the expansion device matches that required to minimize, if not totally eliminate, any flow of refrigerant gas to the evaporator.
If the valve is closed too far, the liquid in the condenser builds up creating the possibility of evaporator starvation, and if the valve is open too far the refrigerant liquid level in the condenser drops causing gas to bypass the condenser and flow to the evaporator. When gas bypass occurs, the compressor must perform more work to support the gas flow. However, no additional cooling capacity is obtained. This decreases the overall chiller operating efficiency.
U.S. Pat. No. 5,809,795, hereafter the '795 patent, discusses one technique for controlling the level of liquid in the condenser. The '795 patent uses a fuzzy logic control algorithm to attempt to control the level of liquid in the condenser. The control in the '795 patent uses a dead band region around the liquid level setpoint in the condenser to limit fluctuations in the system caused by feedback noise from the valve actuation process. However, this dead band region creates a significant control gap in the control logic for those setpoint deviations in the dead band region.
Therefore, what is needed is a fuzzy logic control algorithm to position the expansion valve of a chiller system to better maintain a desired liquid level in the condenser for optimum chiller system operating efficiency.